1. Field of the Invention
The present invention relates to a power supply to provide electrical power to a xenon bulb. More particularly, but not by way of limitation, the present invention relates to a transformerless power supply for a xenon bulb which, in one embodiment, provides a constant programmable power to the bulb.
2. Background of the Invention
Continuous arc xenon bulbs provide bright, stable, daylight balanced light at power levels from a few watts up to tens of thousands of watts. Such bulbs are widely accepted in architectural, entertainment, and medical applications. Typically such bulbs require a moderate DC voltage (on the order of 18 to 150 volts) at a relatively high current for steady-state operation. In addition, a higher voltage is usually provided for starting (usually between 2 and 10 times the operating voltage) along with a very high voltage, short duration ignition pulse (on the order of several kilovolts for a period ranging from a few microseconds to a few milliseconds). This higher start-up voltage and the ignition pulse tend to complicate xenon power supply designs.
Presently, xenon power supplies may be logically divided into two distinct groups: a) those that operate at line frequency, otherwise known as magnetic ballasts; and b) those that operate at higher frequencies, commonly referred to as electronic power supplies. It should be noted that the terms xe2x80x9cballastxe2x80x9d and xe2x80x9cpower supplyxe2x80x9d are often used interchangeably. Magnetic ballasts typically employee a transformer followed by a rectifier and filter capacitors to provide the steady-state electrical power, much like a conventional linear power supply. Magnetic ballasts rely on the inductance of the transformer, or a separate inductor in series with the transformer, to limit the current provided by the ballast. The inductance acts on the line frequency of the AC power supplied to the ballast leading to ballasts which are characteristically large and heavy compared to their electronic counterparts.
Electronic power supplies, on the other hand, typically rectify and filter the incoming electrical power. Solid state switches such as transistors, MOSFETs, IGBTs, or the like, are used to xe2x80x9cchopxe2x80x9d the resulting DC voltage at a relatively high frequency, typically somewhere between 10 kilohertz and 100 kilohertz. A transformer is then used to produce a lower voltage which is again rectified and filtered to provide a steady-state direct current output. The higher frequency provides substantial reductions in the size and weight of the transformer and efficient regulation of the output voltage may be easily achieved by varying the frequency at which switching occurs, the duty cycle provided at the switches, or both. While electronic power supplies are smaller and lighter than their magnetic counterparts, they are also more complex. In addition, electronic power supplies designed to power xenon bulbs above 3600 watts presently stretch the practical limits of the solid state switches employed, resulting in hot components and reduced life of the component parts. Presently, the selection of a particular solid state switch requires balancing switching frequency, and thus the size and weight of the reactive components, against power handling capability.
Thus, magnetic ballasts have dominated the high power xenon field. The term xe2x80x9chigh powerxe2x80x9d as used in conjunction with the present invention refers to xenon bulbs which are designed to consume more than about 2500 watts of electrical power. Practically speaking, short-arc xenon bulbs may presently be produced up to about 20,000 watts while long-arc xenon bulbs of at least 100,000 watts are presently available.
While magnetic ballasts perform satisfactorily in many applications, they are marginal for use in the entertainment industry for a number of reasons. For example, such ballasts often produce xe2x80x9cripplexe2x80x9d at the line frequency or, perhaps, at twice the line frequency. In the United States, this results in 60 Hz or 120 Hz flicker. When a filmed scene is lighted with a xenon powered by such a ballast, xe2x80x9cbeatingxe2x80x9d between the motion picture frame rate and the flicker can result in flicker at a much lower, perceivable rate in the recorded images. In addition, flicker at any rate will totally preclude the use of frame rates higher than the flicker rate. Furthermore, magnetic ballasts designed for these power levels are often too heavy to be moved manually and therefore require undue time and labor for setup and tear down.
While high power electronic power supplies are available, the size and weight of such devices approaches that of magnetic ballasts. Presently, the most palatable solution for the entertainment industry is the ganging of lower power electronic power supplies to supply high power xenon bulbs. xe2x80x9cGangingxe2x80x9d involves the parallel connection of two or more power supplies. To date, the ganging of lower power electronic power supplies has proven reasonably effective up to power levels of 10 kilowatts. Unfortunately, not all electronic power supplies are gangable and, of those that are gangable, load sharing among ganged power supplies is less than perfect. Therefore, it is common for one power supply in a ganged configuration to operate at substantially higher temperature than its co-power supplies, resulting in unreliable operation and premature failure of the over-worked supply. In addition, it has been observed that ganging power supplies may produce substantial ripple, and hence flicker, at rates which are much lower than the switching frequency of the power supplies, thus also raising concerns when used to light a motion picture scene.
Another problem which arises in the use of high power xenon bulbs is inconsistent bulb voltage. First, bulb operating voltage may vary significantly over the life of the bulb. Second, there are significant variations in bulb voltage from bulbs offered by different bulb manufacturers. Finally, bulb voltage varies significantly with the temperature of an individual bulb and, therefore, varies as the bulb heats during use. Neither magnetic ballasts or electronic power supplies presently handle such variations in bulb voltage appropriately. In virtually all instances, the bulb will be operated above or below rated power depending on whether the bulb operating voltage is above or below the voltage for which the power supply was designed. In many respects, an ignited xenon bulb resembles a zener diode, e.g., large changes in current flowing through the bulb result in relatively small changes in bulb voltage. Thus, proper regulation of bulb brightness requires the operation of the power supply in a xe2x80x9cconstant powerxe2x80x9d mode. Typically, presently available electronic power supplies tightly regulate either output voltage or output current, either of which results in inconsistent bulb brightness as the bulb voltage varies.
Additionally, prior art electronic power supplies have utilized a transformer to step down the xe2x80x9cchoppedxe2x80x9d input voltage to a voltage closer to the bulb voltage. Thus used, the transformer may serve a number of purposes. For example: the output power to the bulb is isolated from the power line and from earth ground; the transformer may be included in the oscillator design which drives the solid state switches, as with a relaxation oscillator; the inductive nature of the transformer provides an upper limit on the electrical current; and the transformer provides a reduction in voltage, allowing the switches to operate at a higher duty cycle which improves the power supply""s ability to resolve the output voltage. Unfortunately, the transformer is a large, heavy, and costly component of a high power xenon ballast.
A final consideration in the design of a high power xenon ballast is the apparent phase angle between the incoming voltage and incoming current, otherwise known as xe2x80x9cpower factorxe2x80x9d. Power factor is defined as the cosine of the phase angle between voltage and current in an AC system. Ideally any system connected to an AC power line will exhibit a power factor of one. Generally speaking, a power factor of less than one poses a problem for the utility company, rather than the user of the electrical power, resulting in increased line losses. However, many jurisdictions require electrical products to carry the mark of a recognized testing laboratory and typically the standards applied by such laboratories set limits on the power factor exhibited by electrical devices connected to AC power. Thus, a xenon power supply aimed at a global market will require power factor correction for compliance with such standards. While some xenon power supplies presently include power factor correction, none of these supplies take advantage of a power factor correction scheme which can reduce the size, weight, and cost of downstream components and actually facilitate a transformerless power supply.
It is thus an object of the present invention to provide a transformerless electronic power supply for a xenon bulb.
It is still a further object of the present invention to provide a power factor corrected electronic power supply for a xenon bulb.
It is yet a further object of the present invention to provide a transformerless, power factor corrected high power xenon power supply which weighs substantially less than presently available high power ballasts.
The present invention provides a microprocessor controlled, transformerless, high power xenon power supply which is power factor corrected as to the incoming line. The power factor correction provides a first stage of voltage regulation. A second stage, switching regulator, synchronized to the power factor correction, provides power regulation at a predetermined wattage, regardless of bulb voltage as long as bulb voltage remains within a prescribed range. Synchronization of the power factor correction and the second stage regulator allows a reduction in value, and therefore the size, of the filter capacitors required to reduce ripple to a particular level.
In a preferred embodiment, a programmable microcontroller monitors the output voltage and output current to derive output power. The microcontroller adjusts the duty cycle of a pulse width modulated output, which drives solid state switches of the second stage regulator, to maintain a substantially constant output power.
Preferably, the second stage regulator incorporates one or more current paths depending on the output power desired. Each current path comprises: a solid state switch, i.e. a transistor, MOSFET, IGBT, or the like; an inductor; and a capacitor. The number of current paths employed determines the maximum power output of the power supply. Thus, by way of example and not limitation, if a 4000 watt power supply employed a single current path, a 7000 watt power supply would employ two current paths, and a 10,000 watt power supply would employ three current paths. The individual elements of each current path are therefore no larger than required to attain the maximum output level for a given power supply wattage.
The power factor correction circuit employs a controller which monitors the input current and input voltage, and modulates an output to one or more solid state switches to shape the input current to match the input voltage at a phase angle near zero. Similar to the second stage regulator, the power factor correction provides one or more current paths, depending on the desired output power. Preferably each current path comprises: an inductor connected to a solid state switch in a boost configuration and a diode for summing the outputs of the various current paths into one or more capacitors.
In one preferred embodiment, the inventive high power xenon power supply includes an input to control dimming of the xenon bulb. In a dimming configuration, a maximum voltage (i.e., five volts DC) applied to the dimming input results in the power supply producing the maximum output power. Zero volts applied to the dimming input results in the power supply producing a minimum output power, typically 40% of the maximum power. A voltage in between the maximum and minimum voltages would result in an intermediate output power proportional to the level of the applied dimming voltage.
For starting, the capacitors of the second stage regulator are charged to a starting voltage, typically on the order of 150 volts. An ignition pulse is then triggered by the microcontroller through a conventional ignitor circuit, resulting in a high voltage pulse applied across the xenon bulb. Upon detecting current flow from the second stage regulator, indicating an ignited bulb, the microcontroller begins regulating the output at a predetermined level.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.